TECHNICAL
ABOUT WIND LOADS Russell Harris Technical Manager, AGWA
All buildings are subject to the elements of extreme weather which include high wind speeds. Wind loads on buildings can be substantial and, in many cases, will act as one of the most significant forces that the building will be subjected to.
WIND DIRECTION
Wind load is a dynamic force as it varies greatly in intensity over time. As wind flows around a building, it exerts high pressure on the windward face (face of the building towards the wind) and low pressure (suction) on the leeward face (face of the building against the wind). Leading edges (corners) experience the greatest force due to wind acceleration.
POSITIVE PRESSURE
NEGATIVE PRESSURE (SUCTION)
Wind speed and wind pressure Wind speed and wind pressure are closely related but are not the same. Wind speed is an expression of the velocity of the air moving past a stationary object. It is most commonly measured in metres per second (m/s). Site wind speeds are derived from recorded regional wind speed data, the local effects of terrain (height, topography and shielding) and the relevant annual probability of exceedance. Site wind speeds are the predicted speed of wind over the building site and are determined without consideration of the effects of the building itself (ie, calculated as if the building does not exist). Wind pressure is the force per unit area exerted over a structure by moving air. The pressure varies with the velocity of the air (both wind speed and direction) and the aerodynamic shape and orientation of the structure. Different parts of the same building experience different wind pressures given the same site wind speed. Differential wind pressure is the difference in pressure between the inside and the outside of a building surface (eg, a window). Pressure is measured in pascals (Pa). Regulatory requirements The National Construction Code (NCC) prescribes that a building must perform adequately under all reasonably expected design actions and withstand extreme or frequently repeated design actions, including the effects of wind loads. The NCC references two Australian Standards for the determination of wind loads: •
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AS 4055 Wind Loads for Housing is referenced only in NCC Volume 2, which covers Class 1 and 10 buildings (houses, garages, carport, etc) and provides a simplified set of requirements specifically for houses that meet certain geometric limitations. BUILTVIEW ISSUE 18 Summer 2023
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AS/NZS 1170.2 Structural Design Actions — Part 2 Wind Actions is the primary engineering Standard used in the design of buildings to resist wind actions and applies to all buildings constructed in Australia.
Wind loads for housing For the purposes of AS 4055, a house is defined as a Class 1 or 10 building (as per the NCC) with the following geometric limitations: •
A maximum width of 16 m, with a length not more than five times the width;
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The distance from the ground to the underside of the eave no more than 6m;
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No more than 8.5 m in total height;
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The width of the eaves not exceeding 900 mm;
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With a roof pitch no more than 35°.
